One example of a field effect transistor installed on a semiconductor device is an insulated gate field effect transistor called a “MISFET” (Metal Insulator Semiconductor Field Effect Transistor). Owing to the characteristics of this MISFET that allow it to be readily applicable to high integration, it has been used widely as a circuit element constituting an integrated circuit.
Irrespective of whether it is of an n-channel conductivity type or p-channel conductivity type, a MISFET usually has a channel formation region, a gate insulating film, a gate electrode, a source region and a drain region. The gate insulating film is disposed in an element formation region on the circuit formation surface (main surface) of a semiconductor substrate, and it is formed, for example, of silicon oxide. The gate electrode is disposed over the element formation region of the circuit formation surface of the semiconductor substrate via the gate insulating film, and it is formed, for example, of a polycrystalline film having a resistance reducing impurity introduced therein. The channel formation region is disposed in a region (just below the gate electrode) of the semiconductor substrate opposite to the gate electrode. The source and drain regions are formed in semiconductor regions (impurity diffused regions) which are disposed on both sides of the channel formation region in the channel length direction.
A MISFET having, as a gate insulating film, a silicon oxide film is usually called a “MOSFET” (Metal Oxide Semiconductor Field Effect Transistor). The term “channel formation region” refers to a region in which a current passage (channel) connecting the source region and drain region is to be formed. A MISFET permitting a current flow in the thickness direction (depth direction) of the semiconductor substrate is called a “vertical type”, while a MISFET permitting a current flow in the plane direction (surface direction) is called a “horizontal type”. A MISFET permitting formation of a channel of electrons (conduction path) in the channel formation region between the source region and drain region (below the gate electrode) is called an n-type (or n-channel conductivity type), while that permitting formation of a channel of an electron hole is called a p-type (or p-channel conductivity type).